1. Field of the Invention
The present invention relates to a communication system in which communication between multiple units is transmitted over a communication link common to the units. In particular, the present invention relates to a communication system in which a pair of master units take turns in transmitting messages to and receiving messages from one or more slave units.
2. Description of the Prior Art
Distributed data processing and process control systems generally use a communication path common to the system, such as a communication bus or link for transferring data among the various stations or units. There have been many different communication protocols which have been developed for determining the circumstances, order and priority among the units for communicating over the common communication link. It is desirable to provide reliable communication while minimizing delays in the transmission of data. Communication protocols typically set the rules by which the various units can access the communication link in order to reduce the possibility of two units transmitting at the same time over the link. Such a "collision" can result in a loss or corruption of the data being transmitted by each unit. Most communication protocols, however, achieve this increased reliability at the expense of increased overhead (i.e. additional time consumed communicating bits which control access to the bus, thus reducing time available for communicating data over the bus). As a result, the effective throughput of the system is reduced.